Catalytic dehydrogenation of ethane over mononuclear Cr(III)–silica surface sites. Part 2: CH activation by oxidative addition

Abstract

AbstractModels of Cr(III)–silica were used to study CH activation in ethane by oxidative addition as a possible route to catalytic dehydrogenation. This mechanism involves a formal double oxidation of chromium and a minimum energy crossing point (MECP) was located on the seam between the quartet spin potential energy surface of Cr(III) and the doublet spin potential energy surface of Cr(V). Subsequent to the change of spin state, the CH activation path passes through a transition state on the doublet potential surface, leading to the formation of an ethylhydridochromium(V) complex. This complex represents only a shallow minimum on the potential energy surface and β‐hydrogen transfer to complete the catalytic cycle must therefore take place in the extension of the CH activation step. The combination of a significant activation energy and a small pre‐exponential factor in the rate constant makes CH activation by oxidative addition an unlikely mechanism for dehydrogenation in this system. Copyright © 2005 John Wiley & Sons, Ltd.